Field of the Invention
The present invention relates to an apparatus for measuring light scattering, haze and other related properties produced by non uniform liquid, solid or gas samples, both in their bulk or surface state.
Description of the Related Art
Light that is scattered upon passing through a film or sheet of a material can produce a hazy or smoky field when objects are viewed through the material. Another effect can be veiling glare, as occurs in an automobile windshield when driving facing the sun. Turbidity is a monitoring parameter used in various fields, such as food industry, water contamination control and water purification plants. Turbidimeters are used, for instance, in wine and beer production, in order to measure the total suspended solids value in a liquid sample. Very often, an inspection image is also needed in order to identify different suspended solids in the sample and their concentrations. Another example could be cheese production in which milk coagulation scattering and clustering effects are off-line monitored.
Haze, glare and gloss are scattering-related monitoring parameters also used in roll-to-roll plastic sheet, glass or film production. These parameters are usually measured off-line with different devices, of high cost and large dimensions, in order to detect imperfections and inhomogeneities in the samples. After the imperfection is detected, an image usually follows to determine its nature. It is desirable to measure such image using the same apparatus that is used to evaluate the scattering.
Haze in a film results in a cloudy appearance or poorer clarity of objects when viewed through the film. This is why haze is a parameter that characterizes transparent and translucent films, not opaque films
A Hazemeter measures the haze (light forward scattering) and the light transmitting properties of plastic sheets, film, glass, and liquid products. They are used in many industry fields such as adhesives, Automotive, Ceramics, Chemical, Coatings, Cosmetics, Detergents, Dyestuffs, Food, Glazing products, Printing Inks, Packaging materials, Petroleum, Pharmaceuticals, Plastics, Polishes, Resins, Varnish, Waxes, etc. . . . American Society for Testing and Materials (ASTM) standards establish that haze is the ratio of the light scattered at an angle larger than 2.5 degree and the total light exiting the sample at any angle.
Haze measurement is regulated by the standard ASTM D 1003 “Standard Test Methods for Haze and Luminous Transmittance of Transparent Plastics” and ASTM E 167/ASTM E2387 for haze greater than 30%.
The ASTM procedure to measure haze consists on an easy calculation from four measurements:
                              H          ⁢                                          [          %          ]                =                  100          *                      (                                                            P                  HA                  S                                                  P                  T                  S                                            -                                                P                  HA                  R                                                  P                  T                  R                                                      )                                              (        1        )            
PHAS is the light power corresponding to beams with angles equal or greater than 2.5° emerging from sample. (HA: High Angles)
PHAR is the light power corresponding to beams with angles greater than 2.5° emerging from sample holder plane but without any sample placed in.
PTS is the total light power emerging from sample.
PTR is the total light power emerging from sample holder plane but without any sample placed in.
A light k vector is defined as a magnitude and direction of a light beam: Its magnitude is either the wavenumber or angular wavenumber of the wave (inversely proportional to the wavelength), and its direction is ordinarily the direction of wave propagation.
High k vectors are defined as vectors whose direction forms an angle equal or greater than 2.5° with respect to the direction perpendicular to the sample surface under test.
Low k vectors are defined as those whose direction forms an angle lower than 2.5°.
The standard haze measurement aims to obtain the power ratio between the high k and the total k vectors emerging from the sample. A final correction with the haze reference is mandatory to reduce, if not eliminate, systematic errors.
Every commercial hazemeter that, according to ASTM standards, carries out two separate measurements, of the high k and total k vectors, requires blocking or rejecting the low k vectors. This comes always with an issue: there is a minimum distance between the sample and optical detector element that should exist in order to avoid geometrical errors. This is represented in FIG. 1a. When the distance between the sample and the low k light blocking or rejecting stage is too short (d1), too many high k vectors are blocked (besides the low k vectors). At increasing distances this effect is reduced, e.g. d2 is lower than for d1. In order that this effect is negligible, one has to reach a minimum distance (d3), which depends on the beam size (D). Note that also the blocking stage has to be increased in diameter with the distance because of intrinsic diffraction effects. Note that d3 also sets the resolution of this scheme. The smaller the distance the larger the contribution from k vectors further from 2.5 degree angle. In other words, the larger d3 is, the clearer the separation between k vectors. This is a important drawback because, correspondingly, the device requires a minimum size, sometimes this becoming an unpractical feature, especially for in-line measurements. Commercial hazemeters usually use an integration sphere (see FIG. 1b), which is an optical device that concentrates all the k vectors to one point inside it, with an on/off window. The collimated light source and an integrating sphere must be placed on opposite sides of the sample and must be structurally interconnected in order to preserve the alignment between source and integrating sphere. When the window is open, low k vectors escape from the integration sphere and an optical sensor inside the integration sphere measures only the high k vectors. When the window is closed, the optical sensor measures the total k vectors. In FIG. 1b, distance d3 has been defined as the minimum distance between the sample and the blocking stage in order to separate high from low k vectors. For a hazemeter according to the state of the art, with a beam diameter of 2.5 cm, the minimum d needed applying ASTM D1003 (that is, the needed Integration Sphere diameter) is 28.63 cm. This distance limits the size and configuration of the device.
An alternative way of calculating the haze of a sample is described in WO2010/104699. The method comprises measuring the power of high k vectors and adding it to that of low k-vectors. This method however has the drawback that rays with a k-vector greater than the radius of the sensor will not be detected. The detector must be thus adapted to the dimensions of the sample. A sensor is expensive and thus, having a set of them for different samples results costly.
There is thus a need for a method and device for measuring light scattering with smaller size, lower cost components, making it also possible to be used for in-line measurements.